Created attachment 32224 [details]
Small app to reproduce the problem

If an error is sent as part of some asynchronous processing, for example:

@WebServlet(value = "/async", asyncSupported=true)
public class AsyncServlet extends HttpServlet {
       
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		final AsyncContext context = request.startAsync();
		context.start(new Runnable() {
			@Override
			public void run() {
				try {
					response.sendError(500);
				} catch (IOException e) {
				}
				context.complete();
			}
		});
	}
}

Tomcat 7.0.57 renders its default error page rather than the configured custom error page. The same behaviour is also exhibited in recent 8.0.x releases. It's perhaps worth noting that Tomcat 7.0.55 renders an entirely blank page in this scenario.

My expectation is that the custom error page is used and, FWIW, this is what Jetty does.

I have attached a war that can be used to reproduce the problem. /sync can be used to confirm that the custom error page is working. /async will reproduce the problem described above.